The invention relates to an ultrasound system associated with a probe for imaging ultrasound scatterers in a medium. The invention also relates to an ultrasound diagnostic imaging apparatus, including such a system and a probe, for increasing the signal-to-noise ratio (SNR) in medical ultrasound imaging.
A method and apparatus for improving the SNR in medical ultrasound imaging is described in the patent U.S. Pat. No. 5,984,869 (Chiao and alii). This document relates to a method and an apparatus that increase the signal-to-noise ratio (SNR) in medical ultrasound imaging by using Golay-coded excitation signals. A low SNR causes a limited penetration depth and a small dynamic range of the image, thus lowering the diagnostic information of the ultrasonic image. This document proposes a solution to obtain a good SNR, which consists in increasing the signal energy. By using coded signals (e.g. chirp signals, maximum sequences or Barker/Golay codes) the signal energy is increased by a factor of typically 10-80, depending on the length of the coded signals without increasing the maximum amplitude of the ultrasonic signal.
Coded signals present the characteristics to have a long duration compared to conventional pulse excitation signals. So, one significant problem is that the maximum duration of coded signals is limited by the propagation period of these coded signals, which is calculated as twice the distance between the probe and the area to be imaged divided by the propagation velocity of sound in tissue. Therefore, the use of coded signals with a duration longer than twice the propagation period of these coded signals is not possible. That is the reason why, in the cited document coded signals are only used in high depth areas. Moreover, as the length of coded signals is limited by this propagation duration, the energy that can be carried by a coded signal is limited and consequently, the closer is the area to be imaged, the less the SNR can be improved by using coded signals.
It is an object of the present invention to propose an improved ultrasound imaging system in order to overcome the above-mentioned problem relating to the apparatus described in the cited document. This ultrasound imaging system uses coded signals to enhance the SNR at any depth of the area to be imaged.
An ultrasound imaging system according to the invention is claimed in claim 1.
In the ultrasound imaging system of the invention, the transmitting group transmits an elongated coded signal while the receiving group allows reception of the reflected signal at the same time by angular reflection. The overlap of the transmission and reception duration no longer imposes a limitation on the length of the coded signal used.
In a preferred embodiment of the invention, a first and a distinct second transducer arrays constitute the first and the distinct second groups of transducer elements.
In an advantageous embodiment of the invention, the probe comprises means for ultrasonic insulation to insulate the two distinct groups of transducer elements. Effectively, as the transmitted signal is stronger than the reflected signal, a part of the transmitted signal could be received by the receiving transducer elements masking the reflected signal.
In a preferred embodiment of the invention the nature of coded signals provided by the transmission means is a sequence whose cyclic autocorrelation is a delta function. The convolution of a delta function with the reflector finction of the tissue is the reflector function itself. For instance, M-sequences constitute coded signals that result, after a cyclic autocorrelation, in a short, sidelobe-free powerful peak. According to the present invention M-sequences used are chosen to have a duration longer than the longest propagation period.
In an advantageous embodiment of the invention the nature of the coded signal provided by the transmission means is a xe2x80x9cdoubledxe2x80x9d or xe2x80x9ctripledxe2x80x9d sequence whose cyclic autocorrelation is a delta function. This is realized by a signal doubler that outputs a code of double or triple length. This xe2x80x9cdoubledxe2x80x9d or xe2x80x9ctripledxe2x80x9d signal is transmitted to the probe.
A particular embodiment of the invention comprises correlation means to correlate beam-summed received signals with said coded signals so as to form decoded signals. The correlation of a coded signal which is a xe2x80x9cdoubledxe2x80x9d or xe2x80x9ctripledxe2x80x9d signal with the M-sequence itself results in a pseudo-cyclic autocorrelation. The substitution of the cyclic autocorrelation by a non-cyclic correlation of the signal with a xe2x80x9cdoubledxe2x80x9d or xe2x80x9ctripledxe2x80x9d version of said signal offers good results since one half of the resultant signal is then the same as the resultant signal obtained by using the full cyclic autocorrelation. So, doubled M-sequences are used to receive the favorable result of a cyclic convolution in a non-cyclic case.